This guide covers What Is Epoch Time? | TimeFlow — what it means, how it works, and how to use it. The explanations below are followed by worked examples with real dates and real times, a set of frequently asked questions, and links to the tools that do the calculation for you: the time zone converter, the world clock, and the UTC converter. Where a number is given — an offset, a date, a count — it is computed from the IANA Time Zone Database for the date stated, not copied from a table that may be out of date.

Try the tool: Time Zone Converter — convert any time between any two cities, with the daylight saving rules for the date you choose.

Epoch time — also called Unix time — is the number of seconds that have passed since 00:00:00 UTC on 1 January 1970. That moment is the 'epoch', the zero point from which Unix systems count time. At the time of writing, the number is around 1.79 billion, and it increases by one every second.

The reason computers count this way is that it makes time arithmetic trivial. To find the difference between two moments, you subtract two numbers. To sort events, you sort numbers. To store a date, you store one integer. There are no months, no leap years, no time zones and no daylight saving to worry about — the epoch is defined in UTC, and every moment in history has exactly one number.

The epoch was chosen because Unix was developed in the late 1960s and early 1970s, and 1 January 1970 was a convenient round date. It is arbitrary — any date would have worked — but it is now baked into every operating system, database and programming language in the world.

Epoch time has one famous quirk: the 2038 problem. On 19 January 2038, the number of seconds since the epoch will exceed what a signed 32-bit integer can hold, and old systems that still use 32-bit time will wrap around to 1901. Modern systems use 64-bit time and are unaffected — but the 2038 problem is a reminder that even the simplest way of counting time has an expiry date.

In practice

Epoch time is the time that computers actually use, and it is deliberately boring: a single number that only ever goes up. That is what makes it useful. A calendar date has months of different lengths, leap years and time zones; a count of seconds has none of those problems. When a computer needs to compare two times, or sort a list of events, or measure how long something took, it uses epoch time — and converts it to a human date only when it has to show it to you.

The number is bigger than it looks. One day is 86,400 seconds, one year is about 31.5 million, and the current epoch time is over 1.7 billion. That is why epoch times are usually shown in seconds for humans and stored in milliseconds or microseconds by software: the precision matters when you are measuring things that take less than a second, and a database that stores milliseconds can record an event to a thousandth of a second.

The famous limit is the year 2038. On 19 January 2038, a 32-bit signed counter counting seconds since 1970 will overflow and wrap around to a negative number — the same kind of bug that caused the year 2000 problem. Most modern systems now use 64-bit counters, which will not overflow for billions of years, but old embedded systems — in cars, in industrial equipment, in medical devices — may still be affected. It is a small problem today, but it is real.

The numbers: what 12:00 UTC looks like around the world

The table below shows the local time in eight major cities when it is 12:00 UTC on 15 January 2026. The spread is the whole point of time zones: the same moment is a morning in New York, midday in London, evening in Mumbai and night in Tokyo.

City Local time when it is 12:00 UTC (15 January 2026) UTC offset
New York 7:00 am UTC−05:00
London 12:00 pm UTC+00:00
Paris 1:00 pm UTC+01:00
Dubai 4:00 pm UTC+04:00
Mumbai 5:30 pm UTC+05:30
Tokyo 9:00 pm UTC+09:00
Sydney 11:00 pm UTC+11:00
Los Angeles 4:00 am UTC−08:00

Two things stand out. First, the range: from 04:00 in Los Angeles to 23:00 in Sydney, the same instant covers almost the whole day. Second, the half hour: Mumbai is at 17:30, not 17:00 or 18:00, because India uses a half-hour offset (UTC+05:30). If you convert a time for Mumbai by rounding to the nearest hour, you will be half an hour out. The UTC converter shows the offset and the local time for any city on any date, and the time zone converter converts between any two cities directly.

The offsets change during the year for the cities that observe daylight saving. The same eight cities in January and in July:

City Offset in January 2026 Offset in July 2026 Changes?
New York UTC−05:00 UTC−04:00 Yes — one hour
London UTC+00:00 UTC+01:00 Yes — one hour
Paris UTC+01:00 UTC+02:00 Yes — one hour
Dubai UTC+04:00 UTC+04:00 No
Mumbai UTC+05:30 UTC+05:30 No
Tokyo UTC+09:00 UTC+09:00 No
Sydney UTC+11:00 UTC+10:00 Yes — one hour
Los Angeles UTC−08:00 UTC−07:00 Yes — one hour

New York, London, Paris, Sydney and Los Angeles all change by one hour between January and July; Dubai, Mumbai and Tokyo do not, because they do not observe daylight saving. That is why a conversion done in January can be wrong in July — and why the UTC converter asks for the date.

Key facts

  • UTC never observes daylight saving. It is the fixed reference that every other time zone is measured against.
  • The world uses 38 different UTC offsets, from UTC−12:00 to UTC+14:00 — a span of 26 hours.
  • Nepal is on UTC+05:45, the only 45-minute offset in regular use. India is on UTC+05:30.
  • The Prime Meridian is at 0 degrees longitude, through Greenwich, London.
  • A time written in UTC means the same thing everywhere on Earth, in January and in July.

Frequently asked questions

What is epoch time?

The number of seconds since 1 January 1970 (UTC). It is how computers store time: a single number that only ever goes up, with no months, leap years or time zones to worry about.

What is the current epoch time?

Over 1.7 billion seconds. One day is 86,400 seconds and one year is about 31.5 million seconds.

Why do computers use epoch time?

Because comparing and sorting numbers is easy, and a count of seconds has none of the complications of calendar dates — no months of different lengths, no leap years, no time zones.

What is the year 2038 problem?

On 19 January 2038, a 32-bit signed counter counting seconds since 1970 will overflow and wrap to a negative number. Most modern systems use 64-bit counters, which will not overflow for billions of years.

Is epoch time the same everywhere?

Yes. It is a count of seconds since 1 January 1970 in UTC, so the same epoch time means the same instant in New York and Tokyo. The time zone only appears when you convert it to a human date.

What time is it in Tokyo when it is 12:00 UTC?

23:00 the same day — Tokyo is UTC+09:00 all year, so you add nine hours to 12:00 UTC. In New York it is 07:00 in winter (UTC−05:00) and 08:00 in summer (UTC−04:00). The UTC converter shows the time in any city for 12:00 UTC.

What is the time difference between London and Tokyo?

Nine hours in January 2026: London is UTC+00:00 and Tokyo is UTC+09:00. In July it is eight hours, because London moves to UTC+01:00 while Tokyo stays on UTC+09:00. The time zone converter gives the difference for any two cities and any date.

Related pages

Sources

  • IANA Time Zone Database, via Node.js Intl (Intl.DateTimeFormat with timeZone) — offsets, daylight saving dates and local times in this article were worked out from the IANA database, not typed from memory.
  • TimeFlow's own time-zone data (data/cities.ts, data/dst.ts), computed from the IANA database.
  • Time zone rules and offsets: the IANA Time Zone Database, as applied by this site's tools. Offsets and daylight saving dates on this page are computed from the database for the dates stated.
  • TimeFlow's time zone converter, world clock and UTC converter — the live tools behind the numbers on this page.
  • Time-zone and daylight saving rules are set by governments and can change. For flights, deadlines and meetings that cannot move, confirm the time with an official source.